Three al-Qaeda suspects arrested Wednesday by Spanish police had amassed enough explosives to blow up a bus, and were targeting Spain or other countries in Europe, the country’s Interior Ministry said on Thursday.

­“There are clear indications that the suspects arrested could have been planning an attack in Spain, and or, other European countries,” Interior Minister Jorge Fernandez Diaz said. Sufficient quantities of explosives to blow up a bus were found in their La Linea house, and the material could have been especially dangerous if combined with shrapnel, he noted.

The suspects had been under surveillance “for some time,” before two of them were arrested on a bus heading from Cadiz to Irun. The two were possibly intending to cross into France after spending two months in Spain.

"Police moved to arrest them when it became known that they planned to leave Spain," Diaz said.

A Russian, a Chechen and a Turk were arrested during the operation, the Interior Ministry said. The identities of the three men were not disclosed. Two of the men are suspected Al-Qaeda operatives, with the Turkish man acting as their facilitator. One of the operatives was deemed to be a key member of the terrorist organization.

The Chechen was trained in the Russian Army and was an expert on explosives and poison, according to reports in the Spanish media. All three are reported to have attended a militant training camp in Pakistan; two recently returned from Afghanistan.

Police seized TNT, as well as control instructions for light aircraft and small drones, during the operation. Officers are now studying a suitcase whose contents remain unidentified.

"This is one of the most important operations carried out against Al-Qaeda," the Minister of Interior said. The operation involved close collaboration with intelligence services from Spain's allies, he added.

Spain has intensified its counterterrorism efforts, arresting dozens of suspects in the years following the 2004 Madrid train bombings, which killed 191 people and for which Al-Qaeda claimed credit.

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The three suspected members of Al-Qaeda who were arrested by Spanish police (AFP Photo / Spanish Interior Ministry)